Perco applies pressure to CIPP
Cured-in-place pipe lining is no longer unusual, but employing the technique in the remediation of a pressurised main posed some interesting challenges, writes Nick Sheehan, engineering director of PercoJust over a year ago, Anglian Water announced a £5.5M scheme to increase the sewerage capacity in Wisbech, Cambridgeshire. Implementing the scheme involved refurbishing two pumping stations with the renewal of mains pumping to a wastewater treatment works (WwTW) at West Walton.
While rehabilitation using cured-in-place-pipe (CIPP) lining for gravity drains and sewers is no longer unusual, Anglian Water took the technology a step further by applying it to the pressurised, 450mm and 800mm diameter pumping mains at Wisbech.
Under the management of Anglian's framework partner, Balfour Beatty, work started with the conventional construction of a new pumping main across agricultural fields to West Walton. But open-cut working was ruled out for the relining of the mains. Anglian insisted on a no-dig approach to minimise the effect of road works on the town's traffic and householders.
The existing pumping mains dated from the 1970s and had recently suffered breakage problems. This gave concerns about future viability unless full working capacity was restored. One of the two mains was already out of commission and the relining project meant bringing it back on line.
The poor condition of the mains meant any relining could not rely on the host pipeline for structural support. This ensured the pumping mains would not be compromised by any further deterioration of the older material.
The larger 800mm-diameter main was built of asbestos cement and was initially considered for slip-lining in polyethylene. This approach faced two major problems.
The number of direction changes along the 500m main counted against slip-lining because of the need to excavate at each point to insert large diameter bends. Also, as the main is under pressure, it has to withstand end-loads and there would be considerable difficulty in providing enough pipe anchoring in the ground at these locations. It was virtually impossible to exclude the 800mm main from the relining programme and revert to replacement by excavation, as the route passes directly through a large manufacturing site.
The second existing pumping main also transfers foul sewage to the West Walton WwTW over about 1.6km. At only 450mm diameter, the main is also made of asbestos cement. While it was technically feasible to dig up and replace this main, it was an expensive option. It would have also been inconsistent with Anglian's policy of minimising disturbance.
The potentially much less disruptive option of pipe bursting was explored. This would have allowed a new sewer to be installed, either by jacking in segmental pipes, or by drawing in polyethylene pipe behind the bursting head.
But, due to the proximity of other buried services along parts of the route, the possible risk to these services was considered too high. These techniques would also have resulted in an unacceptably large reduction in the internal pipe diameter, which the EcoCIPP liner avoids by having relatively thin glass-reinforced plastic (GRP) walls.
Had they been straightforward gravity sewers, CIPP would have provided the obvious solution for both mains. But the project team was initially unsure that CIPP could be applied in this case. Balfour Beatty had already experienced many successful pipe-relining operations with Perco's EcoCIPP, UV-cured GRP lining system. Within a short space of time, Perco were able to devise a scheme, based on the same technology, which the project team could accept with confidence.
Part of the EcoCIPP installation process is comparable with other CIPP systems.
Following thorough cleaning and inspection of the drain or sewer, the liner is winched through between manholes and then closed off at both ends. At this stage the liner is very flexible.
It is then inflated with compressed air, which presses the liner against the bore of the existing pipe to produce a tight fit. An important advantage that the EcoCIPP has over conventional liners is that if the liner does get stuck during installation, it can simply and easily be pulled out and reinserted, with no additional excavations or damage to the liner.
But the system differs from most others in its curing method. A UV curing lamp/camera unit is drawn inside the whole length of the liner, hardening the resin and forming a high-strength pipe-in-pipe. An internal foil covering is removed and the new pipe is immediately ready for pressure testing. Not only are curing speeds of 50m/h attainable with the UV system, it also avoids the problem of styrene pollution and odours associated with hot water curing liners.
Styrene is a vital part of the polyester resin used in felt liners. It reacts when the resin cures to create a strong, rigid matrix and also acting as a solvent at the liquid stage, to modify the resin viscosity. As styrene is a volatile, potential carcinogen, safety precautions are taken when handling it.
A problem arises with conventional CIPP liners when they are flushed with hot water to activate the curing process by. Flushing causes styrene to wash out of the liner into the water, which is then normally lost to drain, posing a potential threat to aquatic life.
While the curing water can be tankered away for disposal at a WwTW, it can seriously affect biological sewage treatment processes unless it is diluted to concentrations that are no longer harmful. In contrast, UV curing allows the styrene content to be almost completely bound up in the cured matrix. The UV curing technique of the EcoCIPP liner also avoids the H&S issues relating to the use of large quantities of boiling water used in conventional curing systems.
The high glass fibre content and seamless construction of the liner used by Perco gives it superior strength. At a given wall thickness, the carrying capacity of the liner is 100% higher than that of a typical, laminated needle felt liner, allowing thinner wall sections to be used. Although this would help to overcome any significant loss of internal diameter, Perco were aware that a standard EcoCIPP liner was not the whole solution for a pressure application.
In order to test their anticipated design, the company employed Jason Consultants and in particular, their principal, Dr Dec Downey. Operating worldwide at the forefront of trenchless design, Jason Consultants applied ASTM design calculations to estimate the material thickness required, based on its mechanical properties. With co-operation between liner manufacturer Brandenburger, Jason Consultants and Perco, a liner was designed to cope with the rising main operating pressure of 4bar, a test pressure of 7.5 bar and a working negative pressure of -1 bar, without structural support from the old mains. This solution is now called the PressureCIPP system.
Only one design problem remained: How to seal the ends of the liner against leaking under pressure? Perco immediately contacted PMP, installers of the Amex-10-Seal for internal repairs of leaking joints in accessible pipes. PMP had previously established a similar solution for close-fit lined PE pipes and for non-pressurised CIPP installations.
Following discussions and some design modification, the low profile Amex-10-Seal proved perfectly suitable for the pumping main refurbishments at Wisbech. In installing the seal, a robust section of EPDM rubber spans the joint and is fitted using hydraulic expansion of stainless-steel compression rings. The seal is able to accommodate substantial joint movement and is abrasion resistant.
After submitting plans to Anglian project leader Greg Parish, Perco gained approval and started work in May with Balfour Beatty. The first week saw a total of 350m of 800mm diameter installed, with work progressing quickly on other sections. As the project was phased, the 450mm main was relined in August and as the liner is not thermally cured, it was possible to store it safely on site in wooden crates, for up to two weeks.
High-pressure jetting followed by CCTV surveying was standard preparation for installing the liner, which negotiated bends without any detrimental effect to the system. Initial inspection of the completed installations indicated promising results, which have since been borne out by pressure testing to 7.5bar. In terms of the flow capacity, results are expected to be outstanding. The rehabilitated pipes have retained almost the whole original bore and with the resin-rich surface of the liner providing a shiny inner coating, it is likely that flow has been marginally improved.